1. Technical Field
The present invention relates to methods for manufacturing ferroelectric memory devices having ferroelectric capacitors.
2. Related Art
A ferroelectric memory device (FeRAM) is composed with ferroelectric capacitors, and is a nonvolatile memory that is capable of low voltage and high speed operations. In such a ferroelectric memory device, its memory cell can be composed of, for example, one transistor and one capacitor (1T/1C), such that integration to the level of DRAM is possible. Accordingly, ferroelectric memory devices are highly expected as large capacity nonvolatile memories in recent years.
A ferroelectric capacitor in such a ferroelectric memory device is formed from a lower electrode, a ferroelectric film and an upper electrode. When the ferroelectric capacitor is formed, normally, a layer composed of a material for the lower electrode, a layer composed of a ferroelectric material, and a layer composed of a material for the upper electrode are successively laminated, and these layers are etched and patterned together. As masks that are used for etching, hard masks such as oxide (e.g., SiO2) films are used more in recent years, because resist masks cannot provide sufficient etching resistance (see, for example, Japanese laid-open patent application JP-A-2002-94019). Moreover, when oxide (SiO2) films are not sufficient, the use of titanium nitride (TiN) films as hard masks has been proposed.
When a titanium nitride (TiN) film is used as a hard mask, normally, a second mask pattern composed of SiO2 is formed as a mask for patterning the titanium nitride film, and the second mask pattern is used to form a mask pattern composed of a titanium nitride film. As sufficient etching resistance may not be obtained with the titanium nitride film, normally, the obtained mask pattern composed of a titanium nitride film and the second mask pattern are used together as a laminated hard mask, and ferroelectric capacitor layers are etched together, thereby forming a ferroelectric capacitor.
However, the hard mask described above has a total thickness of about 900 nm, in which the thickness of the SiO2 film is about 700 nm, and the thickness of the TiN film is about 200 nm. On the other hand, a separation between adjacent ferroelectric capacitors is normally about 600 nm. Accordingly, the hard mask described above has a high aspect ratio, and etching of a lower electrode side that is a bottom side of the ferroelectric capacitor may not be favorably etched, and therefore there is a possibility that the adjacent ferroelectric capacitors are conductively connected to each other. In order to prevent conduction between adjacent ferroelectric capacitors, presently, excessive over-etching needs to be conducted. However, when over-etching is conducted excessively, sidewall surfaces of the obtained ferroelectric capacitors become roughened, which makes it difficult to obtain favorable ferroelectric characteristics.